A typical inkjet printing mechanism uses cartridges, often called “pens,” which eject drops of liquid colorant, referred to generally herein as “ink,” onto a print receiving medium. Each pen has a printhead formed with one or more very small nozzles through which the ink drops are fired. To print on a print medium, nozzles of the inkjet printer eject tiny droplets of ink, or dots, during each horizontal pass of the printhead over the print medium to form a row of dots. Each horizontal pass of a printhead over a print medium is called a swath. After each preceding swath, the print medium is incrementally advanced. Through a succession of swaths, images or letters are printed on the print medium.
One form of inkjet printing utilizes fixed “drop weight” ink droplets deposited in a uniform grid. Ideally, each dot has a uniform diameter and each dot location is placed at a uniform distance in each row on the print medium and each preceding row of dots is placed the same distance from the succeeding row of dots. However, minor variations in dot location can produce print defects such as a series of repeated darker vertical bands (vertical striping) that is especially visible on uniform colored images. Vertical striping is caused by repeated patterning of small errors on the printed image. This effect is typically worsened when printing unidirectionally (a print mode in which the printheads deposit ink only while scanning in one direction across the page, and are quiescent on the return scan), as contrasted to bidirectional printing (a print mode where the printheads deposit ink in both scan directions).
Typically, the scanning carriage carrying the pens is scanned across the print media at a substantially constant rate or “swath velocity”. Due to the uniform swath velocity traditionally used, print velocity remains the same for each printing swath. This can create a dynamic resonance within the printing system coinciding with the mechanical system/stiffness of the mechanism. As a result of the constant swath velocity and resonance, vibrations can be induced in the printer which are timed the same on each swath and cause a series of repeated vertical stripe print defects to show up on the printed image.
Constant efforts have been made to decrease print time and increase image quality of images printed with inkjet printing methods. Consequently, a number of costly methods, such as using a combination of different belt materials, have been introduced in an effort to minimize or otherwise hide the vertical striping effects mentioned above.